1. Field of the Invention
The present invention relates to detecting and compensating for interference in GPS signals. More specifically, the present invention relates to a system and method for detecting interference in GPS signals that produce GPS coordinates with unacceptable error.
2. Discussion of Background Information
The use of the GPS network to provide positional coordinates is well known. In summary, a GPS receiver receives different time-stamp signals from a network of satellites, and uses known techniques to derive the latitude and longitude coordinates of the GPS receiver. Signals from at least three satellites are necessary for a ground position, and four for an elevated position.
The accuracy of the resulting GPS coordinates is dependent in part on which satellite signals are used to derive the coordinates. Current GPS receivers are capable of selecting signals from a combination of satellites that provide the most accurate readings. For example, signals from satellites on the horizon are known to contribute to highly accurate coordinates, whereas signals from overhead satellites result in less reliable coordinates.
A weakness of the GPS system is that local interference may block one or more signals from the satellites. Since the satellites on the horizon have the weakest signal, local interference that blocks these signals may force the GPS receiver to use stronger signals from overhead satellites. This can reduce the accuracy of the coordinates derived by the GPS receiver by several hundred meters. Even DGPS, which is a more accurate version of GPS that accounts for various atmospheric conditions, can be adversely affected by local interference.
Recent advancements in miniaturization and manufacturing have led to commercial adoption of GPS devices in vehicles and boats for positional determination. Efforts are also underway to incorporate, and actively use, GPS devices in airplanes for navigation purposes, particularly in landing operations. However, the effect of local interference on the accuracy of GPS coordinates can introduce hazards into aircraft landing procedures, and has been a significant impediment to the adoption of GPS use in aircraft.